In recent years, in order to reduce air pollution, at electric generation facilities utilizing gas turbines, it is demanded to reduce NOx which is included in exhaust gas thereof. NOx in a gas turbine is generated in a combustor which performs combustion behavior in order to rotate the gas turbine. Therefore, conventionally, in order to reduce NOx being generated in a combustor, is employed a combustor having main nozzles which perform combustion (premixed combustion) by mixing the fuel with the air.
By having the main nozzles perform premixed combustion, it is possible to reduce the amount of NOx being exhausted from the combustor. However, combustion state thereof is unstable, and combustion vibrations occur. Therefore, in order to restrain the aforementioned combustion vibrations so as to make the combustion state stable, such a combustor is employed as is further equipped with a pilot nozzle which diffuses and burns the fuel (diffusion combustion). FIG. 20 shows a schematic block diagram of a combustor being provided with a pilot nozzle and main nozzles as mentioned hereinabove.
As shown in FIG. 20, inside a combustor main body 1, a pilot nozzle 2 is inserted into the center thereof. At the same time, main nozzles 3 are inserted so as to be located around the pilot nozzle 2. Then, a pilot cone 4 is installed so as to cover the tip portion of the pilot nozzle 2. Additionally, main burners 5 are installed so as to cover the tip portions of the main nozzles 3. Moreover, pilot swirls 6 are installed around the tip portion of the pilot nozzle 2, and main swirls 7 are installed around the tip portions of the main nozzles 3, so that the pilot nozzle 2 and the main nozzles 3 will be supported.
In a combustor being constructed as mentioned hereinabove, the surrounding area of the tip portion of the pilot nozzle 2 is constructed as shown in FIG. 21. The outer circumference of the tip of the pilot nozzle 2 has a plurality of fuel injection ports 21 installed so as to diffuse and inject the fuel. The fuel which is to be injected by the pilot nozzle 2 will be referred as “pilot fuel” hereinafter. Additionally, the air which is to be supplied to the surrounding area of the pilot nozzle 2 by way of the comb ustor main body 1 (“pilot air”) flows along the inner wall of the pilot cone 4 after passing through the pilot swirls 6. As a result, the pilot fuel being diffused and injected by the pilot nozzle 2 burns, forming diffusion flame (F); and furthermore, a part of the pilot fuel burns and at the same time, high temperature combustion gas from the pilot diffusion flame enters, forming a low-speed flame-stabilizing zone “X” which serves as a flame stabilizing point for the main remixed flame, thereby maintaining combustion.
Additionally, the fuel being injected from the main nozzles 3 (“main fuel”) flows into the main burners 5 together with the air (“main air”), passing through the main swirls 7 and is mixed inside the main burners 5, thereby letting the main fuel and the main air being mixed by the main burners 5 flow out. When an air-fuel pre-mixture which is a mixture of the main air and the main fuel flows out from the main burners 5, the air-fuel pre-mixture is burned toward the inner wall of the combustor main body 1 from the downstream-side tips of the main burners 5, based on combustion in the low-speed flame-stabilizing zone “X.” Herein, “downstream” means the downstream of the fuel and the air flows.
Moreover, as a conventional technology, in order to have a low-speed flame-stabilizing zone “X” formed easily in order to maintain combustion of the air-fuel pre-mixture from the main burners 5, as shown in FIG. 22, is provided a combustor, wherein a pilot cone 4 is formed to be in such a manner as the downstream-side tip thereof projects toward the main burners 5, serving as a pilot cone 4f By forming the pilot cone 4f in the above-mentioned manner, a low-speed flame-stabilizing zone “X” is formed in the vicinity of the downstream-side tip of the pilot cone 4f. 
However, in a combustor being provided with a pilot nozzle 2 and main nozzles 3 as shown in FIG. 21, in order to maintain stability of combustion state thereof, flame-stabilizing effect being supplied by diffusion combustion of the pilot nozzle 2 is necessary. However, because the rate of production of NOx is high when combustion is performed by the pilot nozzle 2, it is necessary to restrain combustion by the pilot nozzle 2 in order to reduce NOx.
Consequently, NOx emission from a combustor is reduced by decreasing the ratio of the fuel being provided to the pilot nozzle versus the entire fuel being provided to the combustor (“pilot ratio”). However, as mentioned hereinabove, when the pilot ratio is low, the flame-stabilizing effect cannot be obtained from the pilot nozzle 2. As a result, combustion vibrations occur, making the combustion state unstable, thereby deteriorating energy efficiency of a gas turbine.
Additionally, stability of combustion can be achieved by forming a low-speed flame-stabilizing zone “X” as shown in FIG. 22. However, in order to promote reduction in NOx further, it is necessary to decrease pilot diffusion flame, but the current low-speed flame-stabilizing zone “X” is not large enough. In addition, because the downstream-side tip of the pilot cone 4f is formed so as to project toward the main burners 5, a stagnant area “Y” where the air-fuel pre-mixture flowing from the main burners 5 forms vortex is formed in the portion at the outlets of the main burners 5 where the pilot cone 4f projects. There is a concern that formation of a stagnant area “Y” might lead to generation of flashback.